A background technology of this technology is a reactor instrumentation system (nuclear instrumentation system) for monitoring reactor power. Examples of the reactor instrumentation system (nuclear instrumentation system) are a local power range monitor (LPRM) and a traversing incore probe (TIP) system. The TIP is used for calibrating a local power range monitor for monitoring power at the time of operating a reactor. A fission chamber is used for the local power range monitor and measures neutrons to output signals (generally, current values). The number of measured neutrons correlates to reactor power, and therefore the reactor power is monitored by the local power range monitor. Generally, a fission chamber is also used for a traversing incore probe (TIP) system. In the case where the local power range monitor is calibrated, the traversing incore probe system is inserted into a position adjacent to a neutron detector for the local power range monitor serving as a target. The traversing incore probe system measures neutrons at such an inserted position to acquire signals in accordance with power, thereby calibrating sensitivity of the neutron detector for the local power range monitor.
Another method is, for example, a method using a gamma thermometer disclosed in JP-A-3-65696 (PTL 1). The gamma thermometer is arranged to be adjacent to a local power range monitor, has no driving mechanism, and is always in an arranged state. The gamma thermometer is different from the TIP in that the gamma thermometer measures gamma rays. The gamma thermometer measures a heating temperature caused by gamma rays to acquire signals corresponding to power in a reactor, thereby calibrating sensitivity of a neutron detector for the local power range monitor.